There is a constant trend to try to decrease the thickness of electronic scales. In the prior art, the electronic scales are provided with 4 feet bearing on the floor, tiles, carpet, etc. Each foot houses a load cell. One example of such configuration is given by document U.S. Pat. No. 4,800,973.
One problem arises when the overall thickness of the scale is decreased, because the height of the feet (i.e. clearance) is not large enough in case the scale is lying on a rather deep carpet, like in a bedroom fitted carpet, or on an uneven floor. If the underbody of the scale touches the floor, this results in an inaccurate, even substantially wrong, weight measurement.
Therefore, some attempts have been done to provide a so-called “sandwich structure” with a rigid top plate and a rigid bottom plate. The structure taught by US20100133016 exhibits shortcomings because the top plate may rub against the bottom plate which causes inaccurate weight measurement.
As a matter of fact, the issue is to prevent relative movement of bottom and top plates in any horizontal direction, while enabling a movement and an accurate weight measurement in the vertical direction.
According to EP0505493, the load cells themselves provide guidance to prevent movement in horizontal directions. However, it turns out that under the stress of a person standing on the scale, there are parasitic stresses which distort the measurement.
Therefore it remains a need to propose a sandwich structure weighing scale which exhibits a thickness of less than 25 mm, and which exhibits an improved accuracy.